Aha, I found the solution here: http://www.macusersforum.com/lofiversion/index.php?t6767.html

Go into /System/Library/Frameworks in Finder and drag WebKit.framework into your project (in the "Frameworks" group). It will show a dialog, the default settings should be fine, just hit OK."
Perhaps this should be included in the book, perhaps as a footnote?

Actually this is in the book. I’ve even highlighted it as a call out because it’s such an easy mistake to make. Check out the Joe Asks in section 2.6 and you’ll find this there. If you don’t think this is a sufficient warning, please let me know.